1. Field of the Invention
The present invention relates to a light diffusing sheet used in a back light unit for a liquid crystal display and a back light unit using the light diffusing sheet.
2. Description of the Related Art
For liquid crystal displays, a back light system in which a back surface of a liquid crystal layer is illuminated to thereby emit light has spread, and the back light unit is provided on a lower surface side of the liquid crystal layer. In general, as shown in FIG. 6, a back light unit 10 comprises a linear lamp 11 as alight source, a light guiding plate 12 placed such that its ends are adapted to those of the lamp 11, a light diffusing sheet 13 provided on a front-surface side of the light guiding plate 12, and a prism sheet 14 provided on a front-surface side of the light diffusing sheet 13. Although one prism sheet 14 is shown in FIG. 6, two prism sheets 14 may be provided such that prism portions 14a of one of the two prism sheets 14 are perpendicular to those of the other.
A function of the back light unit 10 will now be described. Light rays emitted from the lamp 11 enter the light guiding plate 12, are reflected by reflection dots or a reflection sheet provided on a back surface of the light guiding plate 12 and side surfaces thereof, and emanate from a surface thereof. Then, the light rays emanating from the light guiding plate 12 enter the light diffusing sheet 13, are diffused therein, and emanate from a surface thereof. Thereafter, the light rays emanating from the light diffusing sheet 13 enter the prism sheet 14, and emanate from a surface thereof, thereby illuminating the entire surface of the liquid crystal layer (not shown) provided above.
Luminance distributions of the light rays emanating from the surface of the light guiding plate 12, when viewed in a direction perpendicular to the lamp 11 and in a direction parallel to the lamp 11, differ from each other. For instance, as shown in FIG. 7, in some light guiding plates 12, the luminance distribution in the direction perpendicular to the lamp 11 has a high peak diagonally above on an opposite side of the lamp 11, and the luminance distribution in the direction parallel to the lamp 11 has right and left peaks diagonally above that are relatively lower than that of the luminance distribution in the direction perpendicular to the lamp 11.
Specific examples of the conventional light diffusing sheet 13 are as follows:
(a) A binder in which a light diffusing material such as beads is dispersed is applied to a surface of abase sheet to form a light diffusing layer with the light diffusing material dispersed therein on the surface(see Utility Model Application Publication Hei. No.5-73602).
(b) When forming a synthetic resin sheet, a plurality of convex regions and concave regions are formed on a surface thereof by embossing and the like.
Since the above-described conventional light diffusing sheet 13 exhibits a light diffusing ability because of the light diffusing material such as the dispersed beads or the convex regions and concave regions on the surface thereof, the light diffusing ability is the same over the entire surface and in all directions of the sheet.
For this reason, if the diffusing ability of the light diffusing sheet 13 is adapted to either the luminance distribution in the direction perpendicular to the lamp 11 or the luminance distribution in the direction parallel to the lamp 11 and thereby the amount of light emanating in a normal direction (normal direction of respective sheets disposed parallel to one another) in the luminance distribution in one direction is increased, then the amount of light emanating in the normal direction in the luminance distribution in the other direction is not increased. In a case where the luminance distribution of the light rays emanating from the light guiding plate 12 is that shown in FIG. 7, if the diffusing ability of the light diffusing sheet 13 is adapted to the luminance distribution in the direction perpendicular to the lamp 11 that has the high peak diagonally above and thereby a peak light is refracted to some degrees toward the normal direction without greatly dispersing it, then the luminance distribution in the direction parallel to the lamp 11 that is relatively low and has the right and left peaks diagonally above is almost the same as the distribution in which the light rays emanate from the surface of the light guiding plate 12, and consequently, the amount of the light emanating in the normal direction remains small.
Conversely, if the diffusing ability of the light diffusing sheet 13 is adapted to the luminance distribution in the direction parallel to the lamp 11 that is relatively low and has the right and left peaks diagonally above to be thereby enhanced for the purpose of increasing the amount of the light emanating in the normal direction, then a strong peak light in the luminance distribution in the direction perpendicular to the lamp 11 is greatly dispersed, and thereby the amount of light emanating in the normal direction is reduced.
The present invention is directed to solving the above problems, and an object of the present invention is to provide a light diffusing sheet capable of changing a backward and forward diffusing ability and a right and left diffusing ability to be adapted to luminance distribution in a direction perpendicular to a lamp and luminance distribution in a direction parallel to the lamp, and thereby increasing light emanating in a normal direction in either direction, and a back light unit using the light diffusing sheet.
In one aspect of the present invention, a light diffusing sheet for use in a back light unit for a liquid crystal display includes light diffusing means which has a backward and forward diffusing ability and a right and left diffusing ability that differ from each other. The expression xe2x80x9cbackward and forward diffusing abilityxe2x80x9d as used herein refers to a diffusing ability of light rays on a backward and forward plane perpendicular to the sheet surface. The expression xe2x80x9cright and left diffusing abilityxe2x80x9d as used herein refers to a light diffusing ability of light rays on a right and left plane perpendicular to the sheet surface. For convention purposes herein, the backward and forward plane and the backward and forward direction may be considered as extending along the direction B, in the drawings, and the right and left plane and the right and left direction may be considered as extending along the direction A. These directions are examples, however. As illustrated and described, the right and left direction may be considered a first direction and the backward and forward direction may be considered a second direction that is different than the first direction. In an example, the first and second direction may be perpendicular to one another.
With this configuration, the light diffusing means enables to set the backward and forward diffusing ability and the right and left diffusing ability so that they differ from each other. Hence, when a back light unit comprising a)a liner lamp; b) a light guiding plate provided along the lamp for guiding light rays emitted from the lamp to a surface side of the sheet; and c) a light diffusing sheet provided on a surface side of the light guiding plate, uses the above light diffusing sheet as the light diffusing sheet, the diffusing ability of the light diffusing sheet can be adapted to both the luminance distribution in the direction perpendicular to the lamp and the luminance distribution in the direction parallel to it, and the amount of light emanating in the normal direction can be increased in the luminance distribution in either direction. Therefore, in case of the emanating light rays of the light guiding plate represented by luminance distribution the direction perpendicular to the lamp that has a relatively high peak diagonally above on an opposite side of the lamp and luminance distribution in the direction parallel to the lamp that is relatively low and has right and left peaks diagonally above, the diffusing ability of the light diffusing sheet in the direction perpendicular to the lamp is reduced, whereas the diffusing ability in the direction parallel to the lamp is increased. Thereby, the luminance distribution in the direction perpendicular to the lamp has the light that is not diffused so much and reduction of the light emanating in the normal direction is avoided, while the luminance distribution in the direction parallel to the lamp increases the light emanating in the normal direction because of the high diffusing ability. In brief, the light emanating in the normal direction can be increased in either direction. In addition, in case of the light guiding plate that has a smaller viewing angle in the backward and forward direction or in the right and left direction, the light diffusing sheet of the present invention is placed such that the diffusing ability in the direction of the smaller viewing angle becomes higher and the diffusing ability in the opposite direction becomes lower, whereby the viewing angle is efficiently made larger.
As the light diffusing means, there may be formed a plurality of longitudinally divided spindle shaped convex regions or concave regions on the sheet surface with their central axes (longitudinal axes) arranged in a backward and forward direction or in a right and left direction. The term xe2x80x9cspindle shapedxe2x80x9d as used herein denotes a shape in which both ends of a column which has a length in a central axis direction larger than a diameter are gradually thinned, and includes an ellipsoid obtained by rotation using a long axis as a center, or a rotator obtained by rotating a circular arc using its chord as a center, for example. The expression xe2x80x9clongitudinal dividedxe2x80x9d as used herein means that division is conducted along a plane parallel to a central axis of the spindle shape. According to this means, each of the convex regions or the concave regions has a backward and forward cross section different from a right and left cross section in shape. Specifically, the cross section in the direction parallel to the central axis has a curvature of the convex region or the concave region lower than that of the cross section in the direction perpendicular to the central axis. When the concave regions or the convex regions are provided in contact with one another, the number of the convex regions or the concave regions per unit length in the direction perpendicular to the central axes is larger than the number of the convex regions or the concave regions per unit length in the direction parallel to the central axes. For this reason, according to this light diffusing means, the diffusing ability in the direction perpendicular to the central axes of the convex regions or the concave regions is made higher than the diffusing ability in the direction parallel to the central axes of the convex regions or the concave regions, and the central axes are arranged in the right and left direction or in the backward and forward direction to thereby make the backward and forward diffusing ability and the right and left diffusing ability different from each other.
In addition, as the light diffusing means, there may be formed a plurality of quadrangular pyramid shaped convex regions or concave regions with rectangular bottom surfaces on the sheet surface with their long sides arranged in the backward and forward direction or in the right and left direction. According to this means, each of the cross sections in the direction parallel to the long sides of the bottom surfaces of the concave regions or the concave regions has an inclination gentler than that of each of the cross sections in the direction perpendicular to the long sides of the bottom surfaces, and the number of convex regions or concave regions per unit length in the direction perpendicular to the long sides is larger than the number of convex regions or concave regions per unit length in the direction parallel to the long sides. Therefore, the diffusing ability in the direction perpendicular to the long sides of the bottom surfaces of the convex regions or the concave regions is made higher than the diffusing ability in the direction parallel to the long sides of the bottom surfaces of the convex regions or the concave regions, and the long sides of the bottom surfaces are arranged in the right and left direction or in the backward and forward direction to thereby make the backward and forward diffusing ability and the right and left diffusing ability different from each other.
Further, as the light diffusing means, a light diffusing material of a spindle shape can be dispersed in the sheet with central axes thereof arranged in the backward and forward direction or in the right and left direction, or a light diffusing layer in which the light diffusing material of the spindle shape can be dispersed with central axes thereof directed in the backward and forward direction or in the right and left direction is formed on the sheet surface. According to these means, like the above light diffusing means, the cross section of the light diffusing material in the direction parallel to the central axes has the curvature lower than that of the cross section in the direction perpendicular to the central axes, and the number of the light diffusing material in the direction perpendicular to the central axes is larger than the number of the light diffusing material in the direction parallel to the central axes, so that the central axes of the light diffusing material are arranged in the right and left direction or in the backward and forward direction to thereby make the backward and forward diffusing ability and the right and left diffusing ability different from each other.
Still further, as the light diffusing means, there may be formed convex and concave regions on the sheet surface using a hologram technique. The expression xe2x80x9cconvex and concave regions formed using the hologram techniquexe2x80x9d as used herein refers to convex and concave regions obtained by making a light ray having a preferable characteristic as an emanating light ray (light ray having backward and forward luminance distribution and right and left luminance distribution that differ from each other) and a light ray having a characteristic of an incident light ray interfere with each other and by recording data on the sheet surface. According to this light diffusing means, when the incident light ray having the characteristic is applied to the light diffusing sheet, the emanating light ray having the preferable characteristic can be reproduced.
Moreover, there may be formed a plurality of convex prism portions each having a triangular cross section such that they are arranged in the backward and forward direction or in the right and left direction on the surface of the light diffusing sheet. According to the light diffusing sheet, a peak direction in the luminance distribution in the direction perpendicular to the prism portions can be refracted upward by using the prism portions. For this reason, the prism sheet may be dispensed with, or one prism sheet can serve as two prism sheets.
As described above, according to the light diffusing sheet of the present invention, it is possible to achieve the diffusing effect according to the difference between the light collecting characteristic in the direction parallel to the prism portions of the prism sheet or the prism portions of the light diffusing sheet and the light collecting characteristic in the direction perpendicular thereto, and thereby increase the amount of light emanating in the normal direction in either direction.
The back light unit using the light diffusing sheet is capable of reducing luminance nonuniformity of a screen of the liquid crystal display. In addition, the unit is capable of eliminating a peculiar pattern represented by the emanating light rays of the light guiding plate. Further, the unit is capable of significantly reducing reflection of light between constituents, and thereby emanating the light rays upward efficiently.